The present invention relates generally to sublimation printing and is particularly concerned with an electrostatic method and apparatus for effecting such printing.
Printing through the use of sublimatible dyes is known and is rapidly becoming a much utilized process. Generally, the system involves the use of crystalline dye materials which will sublimate, that is, change from a solid to a gaseous phase without becoming liquid. These crystals are ground or pulverized to a fine particle size and then incorporated into a vehicle, such as water, so as to suspend and disperse the dye particles in such a manner so as to form an ink suitable for printing on a paper transfer sheet. The transfer sheet may be imprinted with such inks or by any known commercial process to form the desired image or design in one or more colors. Thereafter, the transfer sheet is placed with its image side against a substrate having a dye receptive surface, such as a urea melamine coating. The sheet and surface are then heated and a thermodynamic equilibrium is set up between the transfer sheet and the receptive surface which is to receive the imprinted design. The time required for effective equilibrium and thus effective transfer to occur is dependent upon known factors including the particular type of receptive coating, the dye concentration, the air gap between the sheet and surface, and the temperature of the various components. The dyes then sublimate into the surface where they condense to form the desired image on the article surface. A particular apparatus and method relating to the field of sublimation printing is disclosed and described in my copending U.S. patent application Ser. No. 895,229, filed Apr. 10, 1978.
It has now been found desirable to provide a paper-less system utilizing the sublimation printing technique. This is accomplished by utilizing a powder coating technology, and specifically, by the utilization of electrostatic means for effecting the coating of a matrix with a finely ground disperse dye powder after which the coated matrix is brought into pressurized contact with the substrate to be printed, wherein sublimation of the desired image or pattern into the substrate is achieved, the substrate being coated with a dye receptive layer to receive the image, as is known in the art.
The utilization of electrostatic means for the application of paints has been known for many years, but the development of electrostatic spraying of powders is more recent, and the development of this technology has been somewhat slow due to the unavailability of suitable, inexpensive plastic powders. One method of applying powders that has been utilized to some extent is the so-called electrostatic fluidized bed process in which the powder resin is applied to a heated object in an open top container. The resin is mixed with compressed air to form a fluid liquid-like mass. The object to be coated is heated above the melting point of the powdered resin and is then immersed into the fluid bed. The resin melts on contact with the heated object to form a coating, and the higher the object is heated and the longer it is immersed in the bed, the thicker the coating. This technique is advantageous in that very thick coatings can be applied in one operation and also parts with blind holes and odd configurations can be coated relatively easily and effectively at a relatively low cost.
In the electrostatic powder spraying process with which the instant invention is concerned, an electrostatic charge is applied to finely ground disperse dye powder, which charged powder is brought into contact with an oppositely charged matrix constructed of a conductive compound so as to be receptive to an electrical charge, whereupon the oppositely charged powder is attracted to the matrix so as to coat same. The coated matrix is then brought into registry with the particle or substrate to be printed, said substrate having a dye receptive coating thereon. The matrix is then forced into pressurized contact with the coated surface of the substrate so as to effect sublimation of the dye pattern into the coated surface of the substrate. This system is particularly applicable to the printing of rigid surfaces, such as hardboard, although not limited thereto. Also, as is true in all powder coating technology, a decided advantage is elimination of solvent carriers necessarily present where transfer paper is being utilized, it being understood that the absence of solvents eliminates the emission of fumes and vapors thereby providing an environment that is virtually free of pollutants. Safety standards are also improved because the hazard of fire is substantially reduced.
Another advantage that results from the use of the powder coating technology of the present invention is the ability to apply dye stuffs which could not be applied by other techniques without difficulty. This is because of the insolubility of some dye stuffs in most solvents. Therefore, many new and different dye stuffs can be applied by powder coating techniques.
Another important feature of the present invention is the fact that the disperse dye powder that is utilized in the system is almost completely used. Expressed differently, excess powder in the system is recirculated and subsequently used, to the extent that approximately 98 percent of the powder supply is effectively used.
Other features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.